1. Field of the Invention
The present invention relates to a thin film transistor, a display device, and a method for driving the display device.
2. Description of the Related Art
As one type of field effect transistor, a thin film transistor is known in which a semiconductor layer which is provided over a substrate having an insulating surface is used for a channel formation region. Techniques are widely known in which amorphous silicon, microcrystalline silicon, and polycrystalline silicon are used for semiconductor layers used in thin film transistors. Thin film transistors have been used, for example, for liquid crystal television devices and put into practical use as switching transistors for pixels of display screens thereof.
A thin film transistor in which an amorphous silicon layer is used for a channel formation region has low electric field effect mobility (approximately 0.4 cm2/V·sec to 0.8 cm2/V·sec) and low on current. On the other hand, a thin film transistor in which a microcrystalline silicon layer is used for a channel formation region has high electric field effect mobility as compared with a thin film transistor in which an amorphous silicon layer is used for a channel formation region. However, the thin film transistor including a microcrystalline silicon layer has high off current as well as high on current and therefore cannot have a sufficient switching characteristic.
A thin film transistor in which a polycrystalline silicon layer is used for a channel formation region has characteristics in that the electric field effect mobility is far higher than those of the above-described two kinds of thin film transistors and the on current is high. Therefore, the thin film transistor including a polycrystalline silicon layer can be used as not only a switching transistor provided in a pixel but also a transistor for a driver circuit which needs to operate at high speed. However, a step of crystallizing a semiconductor layer is needed in a manufacturing process of the thin film transistor in which a polycrystalline silicon layer is used for a channel formation region; thus, there is a problem of high manufacturing cost as compared to manufacturing processes of the above-described thin film transistor including an amorphous silicon layer and thin film transistor including a microcrystalline silicon layer. Further, when laser annealing is employed for crystallizing the semiconductor layer, an area irradiated with a laser beam is small, and thus a liquid crystal panel having a large screen cannot be efficiently produced.
Glass substrates used for manufacturing display panels have been increased in size every year, started from the first generation (e.g., 320 mm×400 mm) to the eighth generation (e.g., 2200 mm×2400 mm) today. It is predicted that glass substrates will be further increased in size from now on, such as the ninth generation (e.g., 2400 mm×2800 mm) and the tenth generation (e.g., 2950 mm×3400 mm). However, no technique has been established yet which is capable of manufacturing a thin film transistor capable of high-speed operation (e.g., the above-described thin film transistor including a polycrystalline silicon layer) over such a large-size glass substrate with high productivity. As the technique by which a thin film transistor capable of high-speed operation is manufactured over a large-size substrate, a technique of manufacturing a thin film transistor in which a microcrystalline silicon layer is used for a channel formation region has been advanced; however, sufficient characteristics of the thin film transistor have not been obtained yet.
A thin film transistor is turned on when a gate voltage (a difference in potential between a source and a gate when the potential of the source is a reference potential) reaches or exceeds a threshold voltage. The threshold voltage is determined depending on the structure of the thin film transistor, the deposition condition of each layer of the thin film transistor, or the like. A technique is known by which, at a position opposite to a gate electrode, a gate electrode (called a back gate electrode) is further provided in order to control the threshold voltage of a thin film transistor (for example, see Non-Patent Document 1). According to Non-Patent Document 1, an insulating film is provided so as to cover a thin film transistor and a back gate electrode is formed using the same layer as a pixel electrode in a region that overlaps with a back channel over the insulating film. The pixel electrode is formed from ITO (indium tin oxide); therefore, the back gate electrode is also formed from ITO. The amount of a drain current (a current flowing between a source and a drain) increases by use of the back gate electrode and accordingly, the amount of on current increases.
On the other hand, a technique for increasing the amount of on current by providing a buffer layer between a gate insulating layer and a semiconductor layer has been disclosed (see Patent Document 1). According to Patent Document 1, the buffer layer suppresses the change in amount of effective charge induced on a semiconductor surface; for example, the buffer layer is formed from a material with a larger band gap width than the semiconductor layer or with higher carrier density than the semiconductor layer. The provision of the buffer layer can also suppress the change in threshold voltage over time and suppress the variation in threshold voltage.
In addition, thin film transistors are required to be capable of high-speed operation. As one way for operating thin film transistors at high speed, a technique by which the minimum channel length is shortened has been disclosed (for example, see Patent Document 2).